The present invention relates to a double flow cage compactor dryer apparatus and method of compacting and drying wastes.
The invention is included in the field of the apparatuses for treatment of industrial wastes, litter, or similar materials.
Disposal of urban solid wastes is an always more felt problem in the modern life. Solutions presently used provide the placement of the wastes in a dump, or, as alternative, the incineration.
Both these solutions have environmental drawbacks. In fact, most dumps quickly reach a saturation level and it is always more difficult to find new areas without protests of the inhabitants of the surrounding zones, worried about the air pollution due to the exhalation of wastes.
Instead, incinerators, even the most technologically advanced for the thermo-valorisation can produce power with efficiency close to 20%, let in the air large amounts of carbon dioxide for each ton of burned waste, beside having the risk due to the creation of toxic products in the fumes, mainly caused by the presence of a high degree of humidity in the refuses preventing to obtain a complete combustion.
Recently, modern plants are widely used, aiming to the contemporaneous volumetric reduction of the refuses and to the inertisation of the putrescent fraction. The goal of this technology is that of obtaining a solid, dry, sterilised and detoxified product that can be sent at the dump or even, in view of its excellent stability features and of its drying state, can be used as material for environmental restoring, i.e. as very good fuel to be used in the thermo-valorisation.
The kind of technical problems faced up and only partially solved by the known plants are mainly connected with the optimisation of the working potentiality and as a consequence to the high practice costs of the apparatuses.
For example, Italian patent for industrial invention N° 1,262,260 describes a two stage system: a first compaction stage, wherein the material to be treated is pressed at room temperature, de-watering the same before soaking up; and a second drying stage, wherein the pre-treated refuses are let within an oven, on the lateral wall of which a box-shaped element is provided along which a thermal carrier fluid passes to release heat to the material. During this second stage, residual water is eliminated from the refuses by evaporation and the vapour produced comes out through the openings realised on the lateral surface of the oven.
This kind of system has different drawbacks: in fact, it is not possible to obtain an even hydraulic set running of the thermal carrier fluid, thus jeopardising the optimisation of the thermal exchange; moreover, oven body resistance properties are not satisfying, also in presence of little pressure stresses, due to the openings on the lateral surface of the heating chamber necessary for the outlet of the vapour.
Still, presence of said lateral openings reduces the contact surface between the wastes and the heat zones, thus reducing the efficiency of the drying stage.
A more advanced solution is described in the EP patent N° 0 663 227 B1, relevant to a drying—compacting apparatus of the cage type for wastes, comprising a cylindrically shaped body containing wastes that must be treated and to sturdy pressing plates to compact the same, said pressing plates being opposed each other and operating as movable basis of said cylindrically shaped body.
With respect to the previous solution, EP patent N° 1 066 490 B1, filed in the name of the same Applicant, has innovative features allowing to obtain the best results. It describes a drying—compacting apparatus of the cage type, having a cylindrically shaped body comprised of longitudinal tubes along the generatrix of said cylindrically shaped body in such a way to be spaced each other, said tubes being coursed by a thermal carrier fluid, acting both as bearing structure of the apparatus and as heat exchange surface between said thermal carrier fluid and the wastes to be subjected td treatment contained within the cylindrically shaped body. Compaction action of the wastes is actuated by two pressure plates opposed each other, acting as movable basis for said cylindrically shaped body.
Heating tubes are constrained each other by retaining rims, set up with a given distance between centres to ensure stability of the tubes forming the cylindrically shaped body stressed by radial thrusts exerted by wastes during pressure and by thermal gradients.
Drying—compacting apparatus further provides outlet means for the steam generated during heating of said wastes, said means being comprised of longitudinal slots between heating tubes.
The system, as the other known solutions, requires a pre-treatment section, wherein wastes are pressed at room temperature to separate the surface liquid fraction comprised of the soaking water. An already de-watered load arrives to the drying—compacting apparatus, from which, by the further pressure—drying treatment, the residual water is extracted and at the same time the volume of the mass is further reduced.
Thus the system works according to a batch mode. In fact, wastes are fed to the pre-treatment section, wherein the first de-watering is carried out and from which they are extracted to be sent to the drying—compacting apparatus, wherein they are subjected to the inertisation before being withdrawn and sent to the storing.
The solution suggested according to the EP patent N° 1 066 490 B1 is such to avoid the hydraulic set running problems of the heating thermal carrier fluid and to solve in a clear way the problem of the outlet of the steam.
Furthermore, the completely original structure of the cage type drying—compacting apparatus is sturdy and perfectly resistant to the thermal and mechanical stresses developed during its operation.
At the same time, drying—compacting apparatus according to EP patent N° 1 066 490 B1 has some features reducing its efficiency.
First of all, thermal mode established within the wastes contained into the cylinder has a logarithmic run; slow growing of the temperature, toward the centre of the mass to be treated, jeopardise the reaching of the design temperatures and their uniform distribution in the product within a short time.
Further, positioning of the tubes along the generatrix of the cylindrical body unavoidably involves that the exchange surface really usable, i.e. really in touch with the material to be subjected to treatment, is only the one faced toward the inner part of the cylindrical body, i.e. a reduced percentage of the whole heating surface of the tubes.
These two limitations would impose opposed solutions, in fact, if on one end, to increase the useful fraction of the exchange surface, it would be necessary to increase the diameter of the cylindrical body comprising the drying—compacting apparatus, on the other end, the logarithmic run of the thermal curve would impose the construction of systems having reduced diameter, that would involve the necessity of realising longer systems or systems comprised of more than one drying—compacting apparatus, working in parallel, to maintain unmodified the same volumetric standard capability.
Finally, a further limitation is due to the difficulties of coordinating the synchronism of the loading, compaction and ejection cyclic sequences from the system, unavoidably bringing to the division of the operative flow into multiple steps spaced by long time intervals and consequently to a reduction of productivity.
Much better would be to have at disposal a less bulky drying—compacting apparatus able to process higher amount of refuses, thus reducing the drying process and allowing to operate continuously.